1. Field of the Invention
The present invention relates to methods of applying voltage to chemical sensors for example, in a throwaway type chemical sensor.
2. Description of the Related Art
A sensor which converts a given chemical substance contained in a test specimen into an electrical signal for measuring a concentration of the chemical substance has been known. This type of the sensor is called a chemical sensor. For the purposes of easy handling and improving the measuring accuracy, improved chemical sensors have been proposed. Some of the chemical sensor of this type have reached the stage of practical use.
One of the known chemical sensors is a blood-sugar sensor for measuring a value of blood sugar in the blood as a test specimen. This blood-sugar sensor uses an enzyme electrode including a hydrogen peroxide electrode and an enzyme electrode using an oxidation reduction enzyme for converting oxygen into hydrogen peroxide. The blood as a test specimen is dropped on the chemical sensor electrically coupled with a measuring instrument, and a blood sugar value of the blood is measured. After the measurement, the sensor (electrode) is disconnected from the measuring instrument to throw away. This chemical sensor of the throwaway type is free from troublesome work after measurement, such as calibration of the calibration curve, washing of the electrodes, and the like. In other words, the chemical sensor can be handled in a maintenance free manner. Thus, the handling of the chemical sensor is remarkably improved. Further, this throwaway chemical sensor does not have the problem of the measuring accuracy deterioration according to an insufficient washing or the like.
FIG. 14 is a perspective view showing a sensor holder with a throwaway chemical sensor set thereto, which was proposed in Japanese Utility Model Application No. 1-141108 filed by the inventors of the present Patent Application. As shown in the drawing, the throwaway chemical sensor is a sensor collected body 50 containing a plural number (e.g., 10) of sensor elements S serially arrayed thereon. This sensor strip 50 is housed in a sensor holder 51. The foremost sensor element of these chained sensor elements of the sensor collected body 50 is exposed at the tip 51a of the sensor holder 51. The exposed sensor element is electrically connected to a not shown measuring instrument through a connector 52 and a cord 53. A test specimen is dropped on the exposed sensor element to measure the concentration of a given chemical substance thereof. After the measurement, a slider 54 of the sensor holder 51 is forwarded to project the used sensor element from the tip 51a of the holder so that the sensor element is cut out and thrown away. When the used sensor is cut out, a new sensor has already been placed at the tip 51a of the holder and ready for the next measurement. In this way, the process of cutting out and discarding the used or old sensor and another measurement using a new sensor are repeated for successive measurements.
It is confirmed that the throwaway chemical sensor succeeds in improving the efficiency and the accuracy of the measurement. However, the throwaway chemical sensor has still the following problems which have to be solved.
It is desirable that the sensor collected body mounted in the holder has a lot of sensors so as to improve a workability, for example, reducing the number of setting the sensor collected body to the sensor holder. In the case of the sensor collected body illustrated in the drawing, an increase of the number of the sensor elements leads to an elongation of the sensor collected body. The length of the sensor collected body that is acceptable for the sensor holder is limited, so that the number of the sensor elements contained in the sensor collected body is also limited. Generally, the calibration value for the sensor is set up for each manufacturing lot, and input to the measuring instrument. Where the exchange of the sensor collected body with a new one is frequent, it is highly probable that an operator mistakenly uses the sensor collected body of another lot, and that calibration values are mistakenly input to the instrument.
Further, the sensor element is bent and cut out every measurement. During this cut-out work, the test specimen may accidentally be attached to other portions than the sensor element. A danger of contamination and infection by the specimen inevitably exists.
General chemical sensors easily lose their function by moisture. For this reason, a moisture-proof must be taken for the chemical sensors, particularly when these are not used. If the holder 51 is designed so as to have a completely sealed structure, it is very difficult to store the sensor collected body after unpacked for long time while keeping its performances.
In addition, enzyme electrodes of the current-detect type which detects a concentration of a glucose (dextrose) contained in blood or urine have been known. Some of the enzyme electrodes are of the throwaway type. An example of the enzyme electrode of this type is disclosed in Unexamined Japanese Patent Publication No. Hei. 2-245650. The enzyme electrode has such a structure that an electrode portion is formed on an insulating substrate, and an enzyme reaction layer is formed on the electrode. The enzyme reaction layer contains hydrophilic high polymer substance, oxidation reduction enzyme, and electron acceptor.
In the enzyme electrode thus structured, when a test specimen solution is dropped on the enzyme reaction layer, the oxidation reduction enzyme and the acceptor are dissolved into the test specimen solution so that the enzyme reacts with the substrate (glucose) in the specimen solution to deoxidize the receptor. The concentration of the substrate in the specimen solution is calculated using an oxidization current value obtained after the enzyme reaction completes. However, in the enzyme electrode thus structured, the oxidation reduction enzyme tends to bond to oxygen. Accordingly, the oxygen dissolved and existing in the specimen solution (this oxygen will be referred to as a dissolved oxygen) antagonistically act, so that the reaction progresses under the influence of the oxygen, and an error is caused in the measurement.
Another type of the enzyme electrode is disclosed in Unexamined Japanese Patent Publication No. Hei. 2-129541. The enzyme electrode disclosed is of the called hydrogen peroxide type. In this electrode, a substrate (glucose) in a test specimen solution reacts with the dissolved oxygen using enzyme as catalyst to generate hydrogen peroxide. Measured is a current generated when the generated hydrogen peroxide is oxidized at the electrode. The current value thus measured is used for calculating the concentration of the substrate in the test specimen solution.
The enzyme electrode of the hydrogen peroxide type uses the dissolved oxygen in the test specimen solution. Therefore, it is not necessary to use the electron acceptor, which is indispensable to the enzyme electrode of the current-detect type. No antagonism between the dissolved oxygen and the acceptor takes place in the test specimen solution, thereby eliminating the measurement error problem by the antagonism. Such an advantageous enzyme electrode of the hydrogen peroxide type has still a following technical problem to be solved.
In the case of the enzyme electrode of the hydrogen peroxide type, the substrate reacts with the dissolved oxygen in a test specimen solution, using enzyme as catalyst. During the reaction process, hydrogen ions are generated. Also when the hydrogen peroxide is deoxidized, hydrogen ions are generated. By the generated hydrogen, the concentration of hydrogen ions is varied in the test specimen solution. When the concentration of hydrogen ions is varied, the reproducibility of a detected current and a detection sensitivity of the sensor become worse in accordance with the pH dependency of the enzyme reaction and the electrode reaction. An accuracy of detecting the concentration of a substance under measurement is degraded, and the resultant calibration curve has a poor linearity.